In the future, it is expected that electronic paper rapidly spreads, which is capable of holding displayed images with no power supply and can electrically rewrite a displayed content. Electronic paper is under development with the aim of providing ultralow power consumption to display images or text in memory even though a power supply is turned off, a reflective display easy on the eyes with no fatigue, and a flexible, low-profile display product with flexibility like paper. It is considered to apply electronic paper to the display components of electronic books, electronic newspapers, and digital signage.
Depending on differences of display types, electronic paper is categorized into such types as an electrophoretic type, a twisting ball type, a liquid crystal display element (liquid crystal display), an organic EL display element (organic electroluminescent display). The electrophoretic type is a type in which charged particles are moved in the air or in a liquid. The twisting ball type is a type in which charged particles each colored with two colors are rotated. The organic EL display element is a self-luminous display element having a structure in which a plurality of thin films formed of an organic material is sandwiched between a cathode and an anode. A liquid crystal display element is a non-self-luminous display element having a structure in which a liquid crystal layer is sandwiched between a pixel electrode and a counter electrode.
Cholesteric liquid crystals are selective reflective liquid crystals of bistable properties using the interference reflection of a liquid crystal layer. The research and development of electronic paper based on liquid crystal display elements proceed with the use of cholesteric liquid crystals. Here, the bistable properties are the properties that liquid crystals show stability in two different alignment states. The cholesteric liquid crystals have properties that two stable states, the planar (planer) state and the focal conic (focal conic) state, are held for a long time after an electric field is removed. In the cholesteric liquid crystals, the incident light is interfered and reflected in the planar state, and the incident light is transmitted in the focal conic state. Thus, because a liquid crystal display panel using cholesteric liquid crystals in a liquid crystal layer is capable of showing light and shade by the selective reflection of incident light in the liquid crystal layer, polarizers are unnecessary. In addition, the cholesteric liquid crystals are also called chiral nematic liquid crystals.
A cholesteric liquid crystal type is predominantly advantageous in the color representation of liquid crystal display elements. The cholesteric liquid crystal type reflects light in a predetermined color by the interference of liquid crystals. On this account, the cholesteric liquid crystal type allows color representation only by placing liquid crystal display panels that reflect lights in different colors in layers. Thus, the liquid crystal display type using cholesteric liquid crystals (here, for convenience, it is referred to as a “cholesteric liquid crystal type”) is predominantly advantageous in regard to color representation as compared with the other types such as the electrophoretic type. In the other types, it is necessary to arrange a color filter tinted into three colors in each pixel in order to conduct color representation. On this account, in the other types, the brightness is about one-third as compared with that of the cholesteric liquid crystal type. Therefore, the other types have a huge problem of improvement of brightness in order to realize electronic paper.
As discussed above, the cholesteric liquid crystal type is a predominant type for electronic paper, which is capable of color representation. However, the cholesteric liquid crystal type has a structure in which three liquid crystal display panels for displaying red (R), green (G), and blue (B) images are placed in three layers in order to realize color representation. Because the cholesteric liquid crystal type has a structure in which three liquid crystal display panels are placed in layers and joined to one another, the cholesteric liquid crystal type has huge problems of a large number of components, complex fabrication processes, and the reliability of joining components.
FIG. 29 schematically shows the cross-sectional configuration of a color display cholesteric liquid crystal display element 100 before. In FIG. 29, for easy understanding, scanning electrode substrates 109b, 109g, and 109r are depicted as rotated at an angle of 90 degrees. FIGS. 30A and 30B show an exemplary state of connecting a liquid crystal display panel 103b for B among liquid crystal display panels 103r, 103g, and 103b for R, G, and B provided in the liquid crystal display element 100 shown in FIG. 29 to a display control circuit board 131. FIG. 30A shows the state in which the liquid crystal display panel 103b for B is seen from the display surface side, and FIG. 30B depicts a cross section cut at line A-A shown in FIG. 30A.
As shown in FIG. 29, the liquid crystal display element 100 has a structure in which three display panels in single colors, the liquid crystal display panels 103b, 103g, and 103r for B, G, and R, are laid on one another and a black opaque layer 119 is arranged on the rear side of the liquid crystal display panel 103r for R. The liquid crystal display panel 103b for B is fixed to the liquid crystal display panel 103g for G by an adhesive layer 117, and the liquid crystal display panel 103g for G is fixed to the liquid crystal display panel 103r for R by an adhesive layer 117.
As shown in FIGS. 29, 30A and 30B, the liquid crystal display panel 103b for B includes a scanning electrode substrate 109b having a plurality of scanning electrodes 121b, a data electrode substrate 111b having a plurality of data electrodes 123b, and a cholesteric liquid crystal layer (B liquid crystal layer 105b) having a thickness of a few micrometers, sealed between the two substrates 109b and 111b, and having a function of wavelength selective reflection. The two substrates 109b and 111b are glass substrates or film substrates. When the two substrates 109b and 111b is flexible film substrates, the thickness of the B liquid crystal layer 105b is sometimes changed by distortion, which degrades display. In order to prevent degraded display caused by the distortion of the two substrates 109b and 111b, the liquid crystal display panel 103b for B has a plurality of wall structures 115b provided between the two substrates 109b and 111b and having adhesive properties. The wall structure is disclosed in Brochure of International Publication No. 06/100713, for example.
One end of each of the scanning electrode 121b and the data electrode 123b is extended and exposed outside a liquid crystal sealing layer for functioning as an external connecting terminal. The liquid crystal sealing layer is a display area surrounded by a sealing agent 113b. Generally, the external connecting terminal of the scanning electrode 121b is connected to the connecting terminal (not shown) of an FPC (flexible printed circuit board) 125b through an ACF (anisotropic conductive film) 149. On the FPC 125b, a liquid crystal drive IC 135b is mounted to drive the scanning electrodes. From the liquid crystal drive IC 135b toward the connecting terminals of the FPC 125b, lead wires 147b are distributed in the same number as the number of the scanning electrodes 121b. 
The external connecting terminal of the data electrode 123b is connected to the connecting terminal of an FPC 127b through an ACF 137. On the FPC 127b, a liquid crystal drive IC 133b is mounted to drive the data electrodes. From the liquid crystal drive IC 133b toward the connecting terminals of the FPC 127b, lead wires 143b are distributed in the same number as the number of the data electrodes 123b. Generally, the liquid crystal display panel 103b for B is externally connected at two places on the scanning electrodes 121b and the data electrodes 123b. 
The liquid crystal drive IC 135b mounted on the FPC 125b is connected to input signal lines 145b having power supply wiring, data wiring, clock signal wiring, and the like. The input signal lines 145b are connected to external terminals on the display control circuit board 131 mounted with a control IC, a power supply circuit (they are not shown), and the like with solders 139. The liquid crystal drive IC 133b mounted on the FPC 127b is connected to input signal lines 141b having power supply wiring, data wiring, clock signal wiring, and the like. The input signal lines 141b are connected to external terminals on the display control circuit board 131 with solders 139. The FPCs 125b and 127b are sometimes connected with a socket instead of the solders 139.
The liquid crystal display panel 103g for G and the liquid crystal display panel 103r for R shown in FIG. 29 have the structure similar to that of the liquid crystal display panel 103b for B, and are connected to the display control circuit board 131.
FIGS. 31A and 31B schematically show cross sections of the liquid crystal display panel 103 connected to the FPC. In order to realize color representation, as shown in FIG. 29, it is necessary to lay the liquid crystal display panels 103r, 103g, and 103b for R, G, and B on one another and bond them together. As shown in FIG. 31B, in the typical technique before (hereinafter, referred to as “prior art 1”), the data electrodes 123r, 123g, and 123b of the liquid crystal display panels 103r, 103g, and 103b for R, G, and B are connected to the FPCs 127r, 127g, and 127b mounted with the liquid crystal drive ICs 133r, 133g, and 133b through the ACFs 137r, 137g, and 137b. In addition, in prior art 1, as shown in FIG. 31B, the scanning electrodes 121r, 121g, and 121b of the liquid crystal display panels 103r, 103g, and 103b for R, G, and B are connected to the FPCs 125r, 125g, and 125b with no liquid crystal drive ICs through the ACFs 149r, 149g, and 149b. In the liquid crystal display element 100, the liquid crystal display panels 103r, 103g, and 103b for R, G, and B are connected to the FPCs 127r, 127g, and 127b and to the FPCs 125r, 125g, and 125b, and then laid on three layers and bonded together. After that, as shown in FIG. 31B, the FPCs 125r, 125g, and 125b are connected to wire lines 155 of an FPC 125 mounted with a liquid crystal drive IC 135 through the ACFs 157r, 157g, and 157b, respectively. The FPC 125 for the scanning electrodes and the FPCs 127r, 127g, and 127b for the data electrodes are connected to the display control circuit board 131 mounted with the control circuit and the like (see FIGS. 30A and 30B).
The liquid crystal display element 100 is a small-sized display element, which is capable of driving the liquid crystal display panels 103r, 103g, and 103b for R, G, and B by single liquid crystal drive ICs 133r, 133g, and 133b, respectively. Even though the liquid crystal display element 100 is small, six places are required to connect the liquid crystal display panels 103r, 103g, and 103b for R, G, and B to the FPCs 125r, 125g, 125b, 127r, 127g, and 127b, three places are required to connect the FPCs 125r, 125g, and 125b to the FPC 125, and thus nine places are required for connections in total before and after placing the panels in layers. In addition, the liquid crystal display element 100 needs seven FPCs and four liquid crystal drive ICs, and the number of components is large. As to a large-sized liquid crystal display element, because a plurality of drive ICs is necessary to drive liquid crystal display panels for R, G, and B, connecting points between the liquid crystal display panels and FPCs are further increased. In addition, when these connecting points are increased, the reliability of liquid crystal display elements is compromised. On the other hand, in the multilayer liquid crystal display element, such a configuration that can reduce the number of FPCs is not realized yet. Because the configuration that can reduce the connecting points is not realized yet, in the multilayer liquid crystal display element, the expenses of component costs and man-hour costs are large and the reliability is also low.
FIG. 32 is a flowchart depicting a general fabrication process of a multilayer liquid crystal display element before using film substrates. FIGS. 33A to 33D schematically show a fabrication process of a multilayer liquid crystal display element before using film substrates. FIG. 33A is a diagram illustrative of a fabrication process in Step S1 shown in FIG. 32, and FIGS. 33B to 33D are diagrams illustrative of fabrication processes in Steps S4 to S11 shown in FIG. 32.
As shown in FIGS. 32 and 33A, on a roll upper film substrate 161, transparent conductors are formed in stripes extended in the longitudinal direction of the upper film substrate 161 to form upper substrate electrodes 163 (Step S1). A large number of the electrode patterns of the upper substrate electrodes 163 are formed on the upper film substrate 161. In addition, on a roll lower film substrate (not shown), transparent conductors are formed in stripes extended in the crosswise direction of the lower film substrate to form lower substrate electrodes (Step S2). The upper substrate electrodes 163 and the lower substrate electrodes are arranged such that they intersect with each other when the upper film substrate 161 and the lower film substrate are bonded together.
Subsequently, depending on the dimensions of the liquid crystal display panel and the final number of panels to be prepared, the upper film substrate 161 is cut into a sheet-like substrate 165 shown in FIG. 33B, a rectangular substrate 167 shown in FIG. 33C, or a separate piece substrate 169 shown in FIG. 33D (Step S3). Subsequently, in the area formed with the upper substrate electrodes 163, in order to keep the thickness of a liquid crystal display cell constant, a cylindrical spacer having a thickness of a few microns is formed (Step S4). Subsequently, the lower film substrate is cut into a sheet-like substrate, a rectangular substrate or a separate piece substrate shown in FIGS. 33B to 33D (Step S5). When the upper film substrate 161 is cut into a shape of the sheet-like substrate 163, for example, the lower film substrate is cut into a sheet-like substrate (Step S5). As described above, the lower film substrate is cut into the same shape as that of the upper film substrate 161. Subsequently, in the area of forming the lower substrate electrodes, spherical spacers are sprayed to hold the thickness of the liquid crystal display cell constant (Step S6). Subsequently, a sealing material (not shown) to seal liquid crystals is formed so as to surround the area of forming the upper substrate electrode 163 (Step S7). In addition, the sealing material may be formed to surround the lower substrate electrodes. Subsequently, the upper substrate and the lower substrate are bonded and joined together to form an empty cell in such a form that the upper substrate electrodes and the lower substrate electrodes intersect with each other and the cylindrical spacers and the sealing material are sandwiched between the upper and lower substrate electrodes (Step S8).
Subsequently, a vacuum filling method is used to fill R liquid crystals for selectively reflecting red light from a filling port of the empty cell (Step S9). When the filling of the R liquid crystals is finished, the filling port is sealed with an end-sealing material (Step S10). Subsequently, when both of the film substrates are cut into the sheet-like substrate or the rectangular substrate in Steps S3 and S5, the substrates are cut into separate pieces shown in FIG. 33D (Step S11). In Step S11, the upper substrate electrodes 163 are exposed in the cut edge of the upper substrate, and the lower substrate electrodes are exposed in the cut edge of the lower substrate. Because the liquid crystal layer has a thickness of a few microns, it is difficult to cut the upper substrate and the lower substrate so as not to damage the upper substrate electrodes 163 and the lower substrate electrodes. On this account, a cut or an opening is provided in advance in the upper substrate and the lower substrate, or the upper and lower film substrates are cut in advance into a separate piece shown in FIG. 33D, not into a sheet-like or rectangular shape shown in FIGS. 33B and 33C. Subsequently, the exposed portions of the upper substrate electrodes 163 and the lower substrate electrodes are formed into connecting terminals, an ACF is used to connect an FPC, and then a liquid crystal display panel for R (component panel R) is completed (Step S13). In addition, the FPC may be mounted with a liquid crystal drive IC, or not.
When a liquid crystal display element capable of color representation is fabricated, by the similar fabrication processes as Steps S1 to S13, a liquid crystal display panel for G (component panel G) connected with an FPC is formed (Step S14). In Step S14, G liquid crystals for selectively reflecting green light are used. Subsequently, by the similar fabrication processes as Steps S1 to S13, a liquid crystal display panel for B (component panel B) connected with an FPC is formed (Step S15). In Step S15, B liquid crystals for selectively reflecting blue light are used.
Subsequently, based on alignment marks formed on each of the liquid crystal display panels for R, G, and B, the individual liquid crystal display panels are aligned with one another from layer to layer, and the panels are bonded together with a photo-curable adhesive or the like (Step S16). In Step S16, for example, the liquid crystal display panels for R and G are placed in layers, and then the liquid crystal display panel for B is laid on the liquid crystal display panel for G. In Step S16, instead of the photo-curable adhesive, an adhesive film may be used. After the liquid crystal display panels for R, G, and B are laid on one another and bonded together, as shown in FIG. 31B, the lower substrate electrodes (data electrodes) of each of the liquid crystal display panels for R, G, and B are connected to the display control circuit board 131 (see FIGS. 30A and 30B) with solder through the FPC (Step S17). In Step S17, as shown in FIG. 31B, the upper substrate electrodes (scanning electrodes) of each of the liquid crystal display panels for R, G, and B are connected to a relay board mounted with the liquid crystal drive IC 135 (scan drive IC) through the ACF, and the relay board is joined to the display control circuit board 131 (see FIGS. 30A and 30B) with solder. Through the fabrication processes described above, a multilayer liquid crystal display element having a narrow picture frame is completed, which is capable of color representation (Step S18).
In the fabrication process of the multilayer liquid crystal display element before shown in FIG. 32, before the liquid crystal display panel are placed in layers, the terminals are joined through the ACF to connect the FPC to each of the liquid crystal display panels so as not to cover the exposed portions of the upper and lower substrate electrodes, and then three liquid crystal display panels are placed in layers. After the panels are placed in layers, the terminals are joined through the ACF to connect the FPC to the display control circuit board or to the relay board. As described above, the fabrication process of the multilayer liquid crystal display element before has a problem that it is necessary to perform the process of again conducting the step of joining the terminals through the ACF, which has been performed. In addition, it is difficult to handle the liquid crystal display panel having the FPCs connected, and a large number of fabrication failures occur, such as misalignment in placing panels in layers, adhesive stains, and joining defects between the FPC and the terminals. Moreover, in the fabrication process described above, because the FPCs are connected to the liquid crystal display panel and then the liquid crystal display panels are placed in layers, it is not possible to prepare multiple panels including the step of layering panels. On the other hand, because defective inspections can be conducted in the stage of preparing a liquid crystal display panel in a singe color, the fabrication process of the liquid crystal display element before has merits that yields are high overall and that liquid crystal display panels can have narrower picture frames.
JP-A-2001-306000 discloses a method in which display panels are collectively connected to each other after the panels are placed in layers in order to prevent a process of again conducting the step having been performed. In the method disclosed in JP-A-2001-306000 (hereinafter, referred to as “prior art 2”), such a structure is configured in which a step exposed portion is provided to the connecting portions of display panels placed in layers, and then an FPC is connected after the display panels are placed in layers. However, because the step exposed portion is provided on the connecting portion, the method has a problem that the picture frame area irrelevant to display is substantially broadened. In addition, because wiring layers overlap one another in the step exposed portions of the connecting portions, the method has a problem that breaks in lines tend to occur, which are caused by pressure in connecting an ACF. In addition, because the dimensions of three types of display panels for R, G, and B are varied, a problem still remains that it is difficult to conduct a so-called preparation of multiple panels in which a large number of multilayered panels are arranged. Such configurations and methods are desired that the picture frame of a display panel is narrow, FPCs can be connected with interconnects after display panels are placed in layers, the numbers of components and connecting points are small, and multiple panels placed in layers can be prepared.
Keizo Takeda, Keiji Matsumoto, Masaki Hasegwa, Kuniaki Sueoka, and Yoichi Taira, Sekiso Kara Hansha-gata Ekisyo Disupurei, Monthly Display, January 2002, PP. 13-17 discloses another method of configuring a multilayer product (hereinafter, referred to as “prior art 3”). Prior art 3 is a multilayer display element in which four glass substrates are used to conduct liquid crystal representation in three layers. The multilayer display element is a color liquid crystal display in which a drive array substrate is provided on the undermost side and transistors for driving pixels on the array substrate are electrically connected to each of liquid crystal cells by through wiring. Three liquid crystal layers can be driven by a single drive array substrate by through wiring. However, because the method of active matrix drive according to prior art 3 needs through wiring holes in the pixels, the method has a problem that the pixel area available for display is narrowed to reduce brightness. In addition, in the multilayer structure of prior art 3, ITO electrodes are provided on the front and back surfaces of the substrate to form a multilayer structure, and then liquid crystals are filled to form a display panel. On this account, when electrodes are formed on the front and back surfaces of the substrate, a problem arises that the electrodes tend to be damaged during processing, which leads to poor production yields. In addition, after three display panels are joined together in three layers, liquid crystals are filled to complete the multilayer display element. Thus, a problem arises that the defects of each of the display panels are multiplied to increase the rejection rate of the multilayer display element. In addition, because prior art 3 requires high temperature processes for joining by through wiring, it is necessary to use glass substrates, which also causes a problem that no film substrate can be used.